This invention is related to implantable probes in the medical industry and, more particularly, to a method for assisting in the implant of a probe.
Although the small subcortical structures targeted during placement of deep brain stimulation (DBS) electrodes can often be visualized on CT or MRI images, the use of imaging alone to guide stereotactic placement is generally considered to be inadequate. CT scans do not clearly visualize important targets such as the internal segment of the globus pallidus in the brain, and do not permit direct use of key coronal planes. MRI images show exquisite anatomic detail, but the effects of magnetic field inhomogeneities, varying magnetic susceptibility, and chemical shifts combine to distort the image and make its stereotactic accuracy uncertain. Although methods have been advocated to minimize MRJ distortion, none have been proven to eliminate all of these difficulties in all conditions. These factors, together with individual anatomic variations, result in the belief of most centers that some form of intraoperative localization is crucial to augment the information derived from imaging.
One of the oldest and most accepted methods of intraoperative localization is that of stimulation, in which the clinical effects of electrical current passed through a test electrode are used to guide position. For example, the proper position for thalamotomy is 1 to 2 mm anterior to a site where stimulation invokes facial and hand response. The accuracy attainable by stimulation is limited, however, by variations in the amount of current spread, differences in individual responses and the inability to accurately measure distances to nearby activated structures.
Electrical impedance has also been used to detect boundaries of subcortical structures. The reported resolution in animal models was excellent but required use of a sharp needle-like probe and electrical shielding. Subsequent use in humans used probes with a more blunt probe tip profile, but these were believed to have less spatial resolution. Although a few clinicians continue to use impedance for intraoperative localization, most centers have abandoned this technique.
Such dissatisfaction for small targets led to development of microelectrode recording (MER), first introduced in humans by Albe-Fessard. In these methods, the specific electrical signals of single neurons, or groups of neurons, are recorded with a fine-wire electrode and used to create an anatomic and physiologic map of key subcortical structures. MER allows definition of important small subcortical nuclei as well as the fine white matter laminae surrounding them. This technique is considered essential by some investigators, and has been used for almost 30 years for reliable and detailed intraoperative guidance.
MER provides two types of information. The first is the identification of areas specific to sensorimotor function (as in globus pallidus targeting) or specific nuclei. The second is the identification of the anatomic extent of the target structure either by mapping the extent of its specific xe2x80x9cneuronal signaturexe2x80x9d or by identifying adjacent boundary structures such as the internal capsule and optic tract. Since most stereotactic targets consist of gray matter surrounded by laminae of white matter, this latter use of MER has been universally used in all centers using MER. For example, one article recently reported the ease and speed of their MER protocol, which emphasized identification of white matter boundaries inferior and medial to the pallidum and dimensions of globus pallidus subcomponents.
There is nevertheless considerable debate regarding the necessity, risks and costs of MER. The requirement of specialized equipment and expertise in neurophysiology adds cost and complexity, while the necessity for painstaking recordings every few microns along tracks several centimeters in length adds time to the procedure, and possibly stress to the awake patient. Several probe tracks are thought necessary to properly use MER adding to the risk of catastrophic hemorrhage, which increases slightly with each track. If DBS placement is believed to be unacceptable without MER, these difficulties may severely limit public access to DBS outside of a few major centers.
While many centers strongly advocate MER, others have questioned the necessity of MER to obtain appropriate clinical results, and MER is not used by the same center that reintroduced pallidotomy into the modern era. Furthermore, a number of groups have reported good results of pallidotomy performed without MER. One group found a 46% improvement in UPDRS motor scores at 12 months following pallidotomy without MER. Another showed a 30% improvement in off-state UPDRS motor scores in their cohort of patients 12 months following pallidotomy without MER. Yet another showed a 44% improvement in off-state UPDRS motor scores after pallidotomy without MER, although the follow-up time was only 3 months. A further one, however, reported five pallidotomies without MER with no improvement. This poor outcome may be due to this report being based upon their early experience, and the use of MRI images without anatomic compensation. These results without MER compare favorably with results of some researchers, which showed clinical improvements of 15, 21, 30 and 30.1%, respectively, at 6 or 12 months after pallidotomy with MER. The improvement in mean off-state UPDRS scores of 58.6% twelve months following pallidotomy with MER is higher than found by most centers.
The need for MER during thalamic procedures has also been questioned, and some centers using MER for pallidotomy do not use this technique during thalamotomy. Furthermore, although localization of the subthalamic nucleus (STN) is achieved my most centers with MER, the ease by which STN is visualized on MRI is cause to question the necessity of MER in this setting.
Whether the use of MER significantly improves clinical results remains controversial. The studies above suggest that the full complement of information available from MER may not be required. This is further supported by the emphasis some centers give to the use of MER to identify gray-white junctions rather than provide detailed maps of neuronal signatures.
MER recordings are typically made by slowly inserting the microelectrode by micrometer increments over a distance of 2 to 3 cm. Each neuron is tested for response to cutaneous stimuli, active and passive limb movements, as well as for its neuronal signature. Such exhaustive testing can take several hours per track and since most protocols require several tracks, most centers use abbreviated protocols to avoid an enormous time requirement. Although typical track times are 30 to 60 minutes, the shortest time per track that has been reported is 20 to 30 minutes. However, after several tracks, even these short protocols add an hour or more to the surgical procedure. Furthermore, these short protocols do not utilize all the data available from MER, and emphasize the identification of gray-white boundaries that might also be obtained from less awkward modalities. Finally, the reports of shorter protocols are from institutions with experience with MER under ideal circumstances. Since MER in practice is quite difficult (factors include fragile probes, the hostile electrical environment of the operating room and the complexity of the recording equipment), it is unlikely that these ideal times can be duplicated by most centers. MER therefore remains a difficult technique that can add significant time to stereotactic procedures.
The amount of risk associated with MER is controversial. Although each extra track adds some risk of hemorrhage, the probability of hemorrhage per track is debated. Several experienced centers have reported low rates of 1 to 2% of hemorrhagic complications. Other groups have not been as fortunate, reporting severe hemorrhage rates of 7%. The discrepancy between these rates from groups early in their experience with the lower rates from more established centers suggests that the risk of MER is real but decreases with experience. Whether the risk of MER attained by these few exceptional centers were matched by the majority of centers performing DBS is debatable.
An explanation for the risk associated with MER is the sharp tip profile of the microelectrode. As noted by one researcher, the blunt tip profile of the NMR probe may confer added safety when compared to sharp electrodes such as used in MER. In primate data from a single laboratory, these investigators reported two intracranial hemorrhages in 60 sharp electrode tracks, while hemorrhages were never encountered in more than 350 electrode tracks with blunt electrodes.
Commercially available systems for MER cost about $100,000. Although systems can be assembled less expensively from individual components, the requirements of high impedance amplification and of a microdrive capable of micrometer increments cannot be avoided. In addition to the technical assistance required to maintain and run the MER recording equipment, interpretation of single cell recording requires neurophysiological expertise. Most centers have found the presence of a trained neurophysiologist in the operating room to be essential, adding to the overall cost.
Probes with sensors mounted at their tip can only xe2x80x9clook aheadxe2x80x9d to interrogate tissue in the probe path. However, the ability to detect gray-white differences perpendicular to the probe path would be invaluable in clinical practice. For example, to ensure adequate proximity to the internal capsule during placement of a thalamic deep brain stimulator electrode, it is often necessary to create a second, more lateral track to map the position of the capsule. If, however, the white matter of the capsule could be detected perpendicular to the original track with a side looking probe, then the proximity of the capsule could be so confirmed and the need for an additional track (with additional morbidity) could be obviated. Current methods of MER are not capable of interrogation perpendicular to the probe axis since the electrodes are straight and cannot be bent.
The present invention disclosed and claimed herein comprises, in one aspect thereof, a method for determining the insertion point for an object at a desired location within tissue of varying types. The parameters of tissue are first measured at an entry point into the tissue from exterior thereof. The parameters of the tissue are then measured periodically along a line extending from the entry point and moving away therefrom. A determination is then made if at least one boundary between tissue types has occurred during the measurement process. The measurements along the line are then compared to a predetermined map of the tissue boundaries. Thereafter, a point on the map is then selected corresponding to the desired location within the tissue when the measured parameters indicate that the measurements are taking place in proximity thereto.